REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment results
Number of reportable segments | segment									2
Fiscal Period Duration									364 days	371 days	364 days
Operating Revenue	$ 516.1	$ 543.9	$ 512.5	$ 474.4	$ 519.3	$ 500.4	$ 467.7	$ 454.2	$ 2,046.9	$ 1,941.6	$ 1,884.9
Operating Income	25.3	61.8	45.9	16.4	37.2	46.7	36.1	34.6	149.4	154.6	196.3
Interest expense, net	(5.4)	(6.2)	(6.3)	(6.3)	(6.7)	(6.0)	(6.5)	(4.9)	(24.2)	(24.1)	(18.5)
Income before Income Taxes	19.9	55.6	39.6	10.1	30.5	40.7	29.6	29.7	125.2	130.5	177.8
Income tax expense	147.0	(21.5)	(15.6)	(3.1)	(10.5)	(15.4)	(11.6)	(11.6)	106.8	(49.1)	(74.8)
Net Income	166.9	34.1	24.0	7.0	20.0	25.3	18.0	18.1	232.0	81.4	103.0
Assets	2,247.5				2,015.5				2,247.5	2,015.5	1,669.8
Capital Expenditures									307.0	179.4	67.8
Depreciation and Amortization									101.2	97.1	83.4
Deferred dry-docking amortization									(46.2)	(38.9)	(23.1)
Depreciations and amortizations including deferred dry-docking amortizations									147.4	136.0	106.5
Intersegment Eliminations
Segment results
Operating Revenue									$ 40.9	$ 20.8	$ 11.3
Ocean Transportation
Segment results
Fiscal Period Duration									364 days	371 days	364 days
Operating Revenue	389.9	419.2	392.7	370.0	406.1	398.0	370.9	366.1	$ 1,571.8	$ 1,541.1	$ 1,498.0
Operating Income	20.7	54.6	39.0	14.5	32.6	43.2	33.9	33.0	128.8	142.7	187.8
Assets	1,937.4				1,722.2				1,937.4	1,722.2	1,601.0
Capital Expenditures									305.3	179.1	67.5
Depreciation and Amortization									93.3	92.6	81.4
Deferred dry-docking amortization									46.2	38.9	23.1
Ocean Transportation | SSAT
Segment results
Equity in earnings from affiliates	28.2				15.8				28.2	15.8	16.5
Equity method investments	93.2				82.4				93.2	82.4	66.4
Logistics
Segment results
Operating Revenue	126.2	124.7	119.8	104.4	113.2	102.4	96.8	88.1	475.1	400.5	386.9
Operating Income	4.6	$ 7.2	$ 6.9	$ 1.9	4.6	$ 3.5	$ 2.2	$ 1.6	20.6	11.9	8.5
Assets	$ 310.1				$ 293.3				310.1	293.3	68.8
Capital Expenditures									1.7	0.3	0.3
Depreciation and Amortization									$ 7.9	$ 4.5	$ 2.0